1. Field of the Invention
The present invention relates to toner materials for imaging by the electrophotographic technique used for printers, copying machines, facsimiles and the like and a process for preparing it. Particularly, the present invention relates to a toner having a multi-layer structure and a process for preparing it.
2. Description of Related Art
In the developing device of an imaging apparatus in an electrophotographic mode using a conventional toner, so-called two-component development has been employed. This is a method for developing an image on an electrostatic image carrier by using a magnetic powder such as iron powder as a carrier, attaching a toner to the surface of the carrier by electrostatic force and transferring the toner on a transporting means in taking advantage of the movement of the carrier by magnetic force (see, for example, C. H. Young & H. G. Greig: R.C.A., 15 (4), 471, 1950). Two-component development has successfully realized a uniform and high density development over the entire range of an image portion, since the carrier acts as an electrode in the neighborhood of the electrostatic image carrier, so that it can correct the shape of the electric lines of force generating from the external surface of the electrostatic image carrier and emphasize a development electric field. The two-component development, however, has disadvantages in that the development apparatus has a short lifetime because of the carrier being susceptible to decomposition and that the development apparatus has a complicated structure for maintaining a constant mixing ratio of the carrier and the toner, and the toner within the development apparatus can be controlled only by complicated sequences. Therefore, the two-component development mode has hitherto been used only for high-speed imaging apparatuses of comparatively high grade.
In order to solve these problems, a small size low-speed imaging apparatus employing mono-component development in which a toner having afforded thereto magnetic properties is used in the absence of a carrier for the development of an image on the electrostatic image carrier (see, for example, U.S. Pat. No. 4,121,931). The toners used in such development include conductive and insulating ones, among which the latter ones are used more popularly in view of the transfer coefficiency of the toner onto recording paper.
Conventional insulating toners used for these development methods are particles comprising a simple substance or blend of a thermoplastic resin such as a styrene-acrylate copolymer or a polyester or a wax in which a color pigment such as carbon black has been dispersed. The particle has a charge control agent or a fluidization improving agent attached to its surface, if necessary, and further has ferromagnetic fine particles such as iron oxide uniformly dispersed into it.
The aggregate of the particles has an insulating property and a specific electric resistance in the range of 10.sup.13 -10.sup.15 .OMEGA..cm. It ordinarily has a dielectric constant of about 3.
As an example taking much account of the dielectric constant in the conventional toners, Japanese Patent Publication No. 6457/89 discloses a toner of which the external surface has been incorporated with at most 5% by weight of fine particles having a high relative permittivity.
On the other hand, in mono-component development, the vicinal electrode effect owing to any carrier cannot be expected so that the electric lines of force generating from the external surface of the electrostatic image carrier cannot be corrected. Thus, the mono-component development has a problem in that only image of a low quality can be obtained when compared with the two-component development because of the so-called edge effect in which the toner at the edge portion of an image is preferentially developed, while the toner at the central portion of the image is developed poorly. In order to obtain a satisfactory development density, it is required that the distance between the electrostatic image carrier and the transporting means, i.e. a development gap, be set up at a very small value and that the value set up be strictly controlled. Parts constituting the developing device must have a high precision in order to satisfy the aforementioned requirement, so that it leads to an extensive increase in cost and thus a stable process cannot be maintained under these circumstances.
The toner disclosed in Japanese Patent Publication No. 6457/89 is effective for preventing the development of a polarizing toner at an image edge portion, while it appears of substantially no effect on the aforementioned problem.
Furthermore, toners having a variety of structures and processes for preparing the toners have been proposed for eliminating the aforementioned problems and improving the functions of toners. For example, there are proposed a toner in which conductive particles are fixed on the surface of a toner in a hot air stream and insulating particles are further fixed thereon in a hot air stream to form a structure having both a conductive layer and an insulating layer on the surface of the toner particles (Japanese Patent Laid-Open Publication No. 257763/88), toners having a multi-layer structure such as microcapsule toners prepared by the polymerization method and used for low-energy fixing (U.S. Pat. 3,080,250, Japanese Patent Publication Nos. 31066/84 and 36934/89), toners of which an external surface is coated with a charge control resin film layer for controlling charge with the use of the high speed impact method in the dry process (Japanese Patent Laid-Open Publication No. 62666/88) or a toner of which the surface has been treated with a resinous finely divided powder in order to obtain anti-blocking properties (Japanese Patent Laid-Open Publication No. 105261/89).
These conventional toners, however, still have some points which needs to be improved.